This invention pertains to the production of pure gaseous hydrogen from a solid hydride at ambient temperatures by processing the solid hydride in an appropriate environment.
During the last decade, a rapidly growing demand for novel energy production technologies, which are alternative to burning basic commodities (e.g., oil, natural gas and coal) and atomic fusion, has significantly increased the importance of hydrogen (H2) as a clean and renewable source of energy. Despite exceptional efficiency attainable by the direct conversion of the chemical energy of hydrogen oxidation into electrical energy (e.g., using a fuel cell), broad use of hydrogen is severely limited by the physical properties of the element (e.g., hydrogen is gaseous at ambient conditions and boils under atmospheric pressure at 20xc2x0 K.). Successful transition to hydrogen-based fuel technology, therefore, requires development of new approaches to transportation, storage, and delivery of significant amounts of pure hydrogen for its use in modern energy generating devices such as, for example, fuel cells. One of the most efficient solutions of the transportation, storage, and delivery problem is to accumulate externally produced hydrogen in the form of compact and safe solid hydride materials (see, e.g., G. Sandrock, in Hydrogen Energy Systems, Utilization of Hydrogen and Future Aspects, Ed.: Y. Yxc3xcrxc3xcm, p. 135, Kluwer Academic Publishers (1995)).
To be economical, solid materials for hydrogen storage should be able to store at least 4.5% of hydrogen by weight (4.5 wt. % hydrogen), i.e., be so-called ultra-high capacity hydrogen storage solids. Such solid hydrides also should easily and controllably release hydrogen at operating conditions of a fuel cell or similar device, preferably at ambient temperature and pressure (see, e.g., Ogden et al., J. Power Sources, 79:143 (1999) and Gunther et al., J. Alloys Compds., 293:889 (1999)). Recently, solid hydrides have been utilized that comprise certain complex derivatives of aluminum hydride (AlH3), usually referred to as complex aluminohydrides (Mxe2x80x2xMy(AlHn)z) where Mxe2x80x2 is Li, Na, or K; M is Mg, Ca, Sr, or Ba; x is 0 or 1; y is an integer between 0 and 3; z is an integer between 1 and 7; and n is an integer between 3 and 6. Two essentially different approaches to extraction of hydrogen gas from these complex aluminohydrides are currently routinely practiced in the art.
The first approach is the thermal decomposition of AlH3 and complex aluminohydrides at temperatures between 60xc2x0 C. and 250xc2x0 C. (see, e.g., Bogdanovic et al., J. Alloys Compd., 253:1 (1997), Zidan et al., J. Alloys Compd., 285:119 (1999), Jensen et al., Int. J. Hydrogen Energy, 24:461 (1999), Bogdanovic et al., J. Alloys Compd., 302:36 (2000), Dilts et al., Inorg. Chem., 11:1230 (1972), Ashby, Adv. Inorg. Chem. and Radiochem., 8:283 (1966), Dymova et al., Russ. J. Coord. Chem., 21:165 (1999), Bastide et al., Stud. Inorg. Chem., 3:785 (1983), International (PCT) Patent Application WO 99/24355, and German Patent Application DE 19,526,434). One of the most commonly used solid hydrides has been lithium aluminohydride (LiAlH4). In the case of the pure LiAlH4, the thermal decomposition proceeds in two steps according to Equations 1 and 2 set forth below, and enables generation of a total of 7.9 wt. % hydrogen between 160xc2x0 C. and 230xc2x0 C. (see, e.g., Dilts et al. (1972), supra, Dymova et al. (1999), supra, Bastide et al. (1983), supra, DE 19,526,434 and U.S. Pat. No. 5,882,623):
xe2x80x833LiAlH4=Li3AlH6+2Al+3H2 160-180xc2x0 C. 5.26 wt. % H2 xe2x80x83xe2x80x83(1)
Li3AlH6=3LiH+Al+1.5H2 180-225xc2x0 C. 2.64 wt. % H2 xe2x80x83xe2x80x83(2) 
Although the extraction of hydrogen from a solid hydride enables generation of significant amounts of pure hydrogen gas, its operating parameters are dependent on the thermal stability of the corresponding hydride. For example, sodium aluminohydride (NaAlH4) decomposes at a different temperature and yields smaller amounts of hydrogen per unit weight of the hydride (see, e.g., Dilts et al. (1972), supra). As a result, the thermal decomposition of aluminohydrides cannot be used for hydrogen gas generation at temperatures below the decomposition temperatures of corresponding hydrides, i.e., below approximately 60xc2x0 C. for AlH3 and about 80-150xc2x0 C. for other modified complex aluminohydrides (see, e.g., Bogdanovic et al. (1997), supra, Zidan et al. (1999), supra, Jensen et al. (1999), supra, and Bogdanovic et al. (2000), supra). Certain reduction of decomposition temperatures of metal hydrides is achieved by their mechanical pretreatment followed by thermal decomposition (see, e.g., U.S. Pat. No. 5,882,623). The decomposition of metal hydrides using their heating with microwaves or ultrasound also has been reported (see, e.g., U.S. Pat. No. 5,882,623). A disadvantage of the thermochemical approach is that it may be difficult to precisely control the rate of hydrogen delivery, i.e., to rapidly increase or reduce the temperature of a large quantity of the hydrogen storage material to initiate or terminate the flow of hydrogen gas.
The second approach, which enables release of hydrogen gas from aluminum hydride and complex aluminohydrides at ambient conditions, involves the chemical treatment of the solid hydride with stoichiometric amounts of appropriate liquid or gaseous reagents, such as water (see, e.g., Kong et al., Int. J. Hydrogen Energy, 24:665 (1999), Lynch et al., Proc. Intersoc. Energy Convers. Eng. Conf., 33rd IECEC217/1-IECEC217/6, (1998), Breault et al., Proc. Power Sources Conf., 39th, 184: (2000), Cotton et al., Advanced Inorganic Chem., 6th Ed., New York, Wiley (1999), and Canadian Patent No. 2,028,978), alcohols (see, e.g., Cotton et al. (1999), supra), or ammonia (see, e.g., Lynch et al. (1998), supra). However, a significant disadvantage of this method is inevitable contamination of the produced hydrogen with the vapor of the used reagent. Therefore, chemical treatment of aluminohydrides is unsuitable for generation of pure hydrogen gas.
Because of these limitations, a more efficient method to produce pure gaseous hydrogen is needed. To the best of our knowledge, all known hydrogen storage media with the storage capacity exceeding 4.5 wt. % hydrogen, such as magnesium hydride (MgH2)- and aluminum hydride (AlH3)-based materials, in pure form, are capable to release hydrogen only at temperatures exceeding 100xc2x0 C. (see, e.g., Bogdanovic et al. (1997), supra, Zidan et al. (1999), supra, Jensen et al. (1999), supra, Bogdanovic et al. (2000), supra, Kostanchuk et al., Russian Chem. Rev., 67:69 (1998), Bifehoom et al., J. Less-Common Metals, 74:341 (1980), Friedlmeier et al., J. Alloys Compds., 292:107 (1999), Hammioui et al., J. Alloy Compds., 199:202 (1993), and Ivanov et al., J. Less-Common Metals, 13:25 (1987)). Hence, a need remains for the possibility of low-temperature (less than 100xc2x0 C.) high-capacity (at least 4.5 wt. %) production of pure gaseous hydrogen. Such a method of hydrogen generation is of interest for technologies that consume pure gaseous hydrogen as a fuel, including energy conversion technologies such as fuel cell technology.
The present invention provides such a method. These and other advantages of the invention, as well as additional inventive features, will be apparent from the description of the invention provided herein.
The present invention provides a method of extracting hydrogen from a solid hydride. The method comprises processing the solid hydride at ambient temperature in the absence of chemical treatment such that hydrogen is released from the hydride. The gaseous hydrogen is then collected. In preferred embodiments, the processing comprises mechanical processing in the presence of a catalyst. Such methods allow for the low-temperature, high-capacity production of pure gaseous hydrogen.